1. Field of the Invention
The present invention generally relates to an aluminum/ceramic bonding substrate and a method for producing the same. More specifically, the invention relates to an aluminum/ceramic bonding substrate wherein an aluminum member is bonded to at least one side of a ceramic substrate, and a method for producing the same.
2. Description of the Prior Art
In recent years, power modules are used for controlling heavy-current for electric vehicles, electric railcars, machine tools and so forth. In a conventional power module, a metal/ceramic insulating substrate is fixed to one side of a metal plate or compound material called base plate by soldering, and one or a plurality of semiconductor chips are fixed to the metal/ceramic insulating substrate by soldering. On the other side (reverse) of the base plate, a radiating fin or cooling jacket of a metal is mounted via a thermal conduction grease by means of screws.
Since the soldering of the base plate and semiconductor chip(s) on the metal/ceramic insulating substrate is carried out by heating, the base plate is easy to warp due to the difference in coefficient of thermal expansion between bonded members during soldering. Heat generated from the semiconductor chip(s) passes through the metal/ceramic insulating substrate, solder and base plate to be radiated from the radiating fin or cooling jacket to air or cooling water. Therefore, if the base plate warps during soldering, when the radiating fin or cooling jacket is mounted on the base plate, the clearance therebetween increases, so that there is a problem in that the heat sink characteristic of the base plate extremely deteriorates.
In order to solve such a problem to enhance the reliability of the metal/ceramic insulating substrate, there is proposed a metal/ceramic circuit board which uses a base plate of aluminum having a very low yield stress, e.g., a metal/ceramic circuit board wherein a base plate of aluminum or an aluminum alloy having a proof stress of 320 MPa or less and having a thickness of 1 mm or more is bonded directly to a ceramic substrate by a so-called molten metal bonding method (see, e.g., Japanese Patent Laid-Open No. 2002-76551).
In order to decrease the yield stress of aluminum, it is required to increase the purity of aluminum. However, in the molten metal bonding method, it is difficult to control the grain size of aluminum, so that the obtained grain size of aluminum is a large grain size of 10 mm or more. If the grain size of aluminum is so large, the grain size distribution of aluminum varies. Thus, cracks are easily produced in the ceramic substrate after heat cycles, and the behavior of warpage of the aluminum base plate varies due to heat when semiconductor chips are soldered thereon.
In recent years, as an insulating substrate for a reliable power module for controlling heavy-current for electric vehicles, machine tools and so forth, there is used an aluminum/ceramic bonding substrate wherein an aluminum member is bonded to a ceramic substrate.
However, in a conventional aluminum/ceramic bonding substrate, if an aluminum member is plated with nickel or the like to bond thereon a heat sink plate, such as a copper plate, via solder, there are some cases where cracks are produced in the solder by heat cycles, so that the heat sink characteristic of the heat sink plate deteriorates. It is considered that such solder cracks are produced in the relatively weak solder layer by stress which is produced on the bonding interface by heat cycles due to the difference in coefficient of thermal expansion between the aluminum member and the solder.
In order to eliminate such a problem, it is known that an aluminum alloy member of an aluminum-silicon alloy is used as the aluminum member (see Japanese Patent Laid-Open No. 2002-329814).
However, when an aluminum alloy member of an aluminum-silicon alloy, particularly an aluminum alloy containing 0.1 to 1.5 wt % of silicon, is bonded to a ceramic substrate by the molten metal bonding method, if the current cooling method is used, it is very difficult to decrease the grain size of the alloy by optimizing usual cooling conditions, and it is very difficult to control the formation of a solid solution of silicon in aluminum. Thus, there is a problem in that the enriching section of silicon exists in the aluminum alloy member. In particular, it is considered that the eutectic of Al—Si or a composition approximating it exists in the grain boundary portions. Thus, there are some cases where hot cracks are produced in the aluminum alloy member during the cooling process, so that the aluminum/ceramic bonding substrate does not function as a normal aluminum/ceramic bonding substrate. In addition, if etching is carried out for forming a circuit on the aluminum alloy member, there are some cases where the enriching section of silicon remains on the surface of the ceramic substrate without being dissolved, so that it is not possible to obtain the desired insulating characteristics of the aluminum/ceramic bonding substrate.